This is a really bad problem to find yourself in, as you have already concluded. You can't be worried about whether your magazine is fully seated, or, for that matter, even there in a self-defense situation.
My XD-40 service has an ambidextrous magazine release, too, and it fits in my SuperTuck Deluxe kydex holster in a very similar fashion to yours. Never in the years I have been carrying it have I had an experience with the magazine becoming unseated or ejecting, and I, too, fumble with seat belts, take naps, and put myself in all sorts of contorted positions from time to time.
Could it be that your magazine release is just too easily activated? I carry Condition 1 and it takes a rather large amount of concentrated effort to release the magazine in the XD.
XD-40 service, XD-9sc, member GeorgiaCarry.Org, National Rifle Association, Gun Owners of America, North American Hunting Club, Rocky Mountain Elk Foundation